Field of the Invention
This invention relates to temperature compensated crystal oscillators and more particularly to accounting for temperature-dependent hysteresis in the frequency response of the crystal.
Description of the Related Art
Crystal oscillators are frequently used to provide a stable frequency source. However, crystal oscillators' frequency stability is affected by temperature and thus temperature compensated crystal oscillators (TCXOs) are utilized to make the frequency supplied by the TCXO more stable in the presence of temperature variations.
FIG. 1 illustrates a high level block diagram of a TCXO 100 that includes a crystal oscillator (XO) 101 (including a crystal and oscillator sustaining circuitry) and a fractional-N phase locked loop (PLL) 103 that receives as an input the crystal oscillator output and supplies a PLL output signal 104. A temperature sensor 105 senses temperature and provides temperature data to a temperature compensation circuit 107 that in turn causes a feedback divider control circuit 109 to adjust the feedback divider 111 to correct for the expected frequency variation resulting from temperature variation. The temperature compensation circuit 107 may utilize a look-up table mapping the sensed temperature to the frequency variation or a polynomial extracted from the sensed temperature and measured frequency variation profile. The temperature compensation values in the look-up table or the polynomial coefficients may be based on device characterization in which the TCXO was heated to various temperatures in order to determine the appropriate compensation factors.
While FIG. 1 provides one way to temperature compensate a crystal oscillator, further improvements in TCXOs are desirable to provide even more stable output signals.